1. Field of the Invention
The subject invention generally relates to an encapsulated particle. More specifically, the subject invention relates to an encapsulated particle that includes a polyurethane layer disposed about a core particle and that is used as a controlled-release fertilizer.
2. Description of the Related Art
Encapsulated particles that are used as controlled-release fertilizers are known in the art. Specifically, the encapsulated particles include layers disposed about core particles. More specifically, the layers that are disposed about the core particles include polyurethane layers. A thickness and external integrity of the polyurethane layers limit a rate that the core particles dissolve in a soil that includes water and moisture.
More specifically, prior art encapsulated particles include a core particle selected from a group of fertilizer particles. The disadvantages of using the prior art encapsulated particles include inconsistent external integrity and thickness of the polyurethane layers resulting in a very fast rate that the core particles dissolve in the soil. As is known in the art, the very fast rate that the core particles dissolve in the soil leads to phytotoxicity. Additional disadvantages of using the prior art encapsulated particles include an inability to effectively customize the thickness of the polyurethane layer disposed about the core particle and the requirement of expensive and perishable manufacturing components, such as castor oil. Castor oil is utilized for the production of the polyurethane layers serving as a polyol that is reactive with an isocyanate to form the polyurethane layers.
Specifically, castor oil is subject to unpredictable market price fluctuations and unpredictable quality control. Further, castor oil is perishable and thus is not suitable for long term storage and use in mass production of encapsulated particles. Still further, castor oil contains double bonds in its lipid structure and is prone to lipid oxidation. Lipid oxidation occurs when the double bonds in the castor oil react with oxygen to form peroxides and change the chemical nature of the castor oil. Finally, castor oil is not aromatic. When serving as a polyol that is reactive with an aromatic isocyanate to form the polyurethane layers, castor oil is not completely miscible with the aromatic isocyanate due to a lack of aromaticity, and thus, is not suitable for use.
Most importantly, the primary disadvantage of the prior art encapsulated particles includes a tendency to exhibit polyurethane layers that include defects. Defects in the polyurethane layers result from an incomplete miscibility between an isocyanate and a polyol that is reactive with the isocyanate to form the polyurethane layers. For example, when an organic, non-aromatic polyol is combined with an aromatic isocyanate, miscibility may not be complete. Rather, the organic, non-aromatic polyol could react with the aromatic isocyanate only at their interface.
Incomplete miscibility between the aromatic isocyanate and the non-aromatic polyol subsequently leads to polyurethane layers that include defects such as pits and depressions. When the polyurethane layer that includes defects is disposed about the core particle, the pits and depressions allow water and other liquids to permeate the polyurethane layer and rapidly dissolve the core particle. To cure the defects, multiple polyurethane layers must be disposed about the core particle resulting in a time consuming and expensive process.
Many different layers can be disposed about core particles. U.S. Pat. No. 5,538,531 to Hudson discloses a plurality of water insoluble, abrasion resistant layers disposed about a core particle that includes a controlled-release fertilizer. A first layer is disposed about the core particle and includes a polyurethane derived from the reaction product of an aromatic isocyanate and a non-aromatic polyol that is reactive with the aromatic isocyanate. A second layer, formed from an organic wax, is disposed about the first layer to cover any defects in the first layer and prevent water and other liquids from permeating the first layer and rapidly dissolving the core particle. The '531 patent does not disclose the use of a polyol derived from an aromatic amine-based initiator.
Similarly, U.S. Pat. No. 6,663,686 to Geiger and U.S. Pub. Nos. 2004/0020254 and 2004/0016276 to Wynnyk, all assigned to Agrium® Inc. of Calgary, Alberta, also disclose a polyurethane layer disposed about a core particle. The '686 patent and the '254 and '276 publications disclose the use of aromatic isocyanates including diphenylmethane diisocyanate, toluene diisocyanate, and mixtures thereof. Additionally, the '686 patent and the '254 and '276 publications disclose the use of non-aromatic polyols including castor oil and hydrogenated castor oil. Yet, neither the '686 patent nor the '254 and '276 publications disclose the use of a polyol derived from an aromatic amine-based initiator.
Yet, the controlled-release fertilizers disclosed in the '686 patent and the '254 and '276 publications are not the sole prior art. U.S. Pat. No. 3,475,154 to Kato discloses a polymer layer disposed about a coated pellet. The polymer layer includes the reaction product of active hydrogen, in the form of polyols and polyamines, and an aromatic isocyanate. The '154 patent does not disclose the use of a polyol derived from an aromatic amine-based initiator.
Finally, U.S. Pat. No. 3,264,089 to Hansen and U.S. Pat. No. 4,711,659 to Moore disclose a plurality of polyurethane layers disposed about a core particle. The polyurethane layers include the reaction product of an aromatic isocyanate and a polyol. In both the '089 and '659 patents, the aromatic isocyanate includes methylene diphenyl diisocyanate, toluene diisocyanate, and mixtures thereof. Additionally, in both the '089 and '659 patents, the polyol includes polyether diols and polyols. Further, in the '659 patent, the polyol involves reactions with amine-terminating groups. Yet, neither the '089 patent nor the '659 patent disclose the use of a polyol derived from an aromatic amine-based initiator. Specifically, in the '659 patent, the polyol reacting with the amine-terminating groups is not equivalent to a polyol derived from an aromatic amine-based initiator. Namely, in the '659 patent, the polyol that includes amine terminating groups is not aromatic and therefore is not completely miscible with aromatic isocyanates. Conversely, the polyol derived from an aromatic amine-based initiator is terminated in an alkyl group and not an amine group. Additionally, the polyol derived from an aromatic amine-based initiator includes amine functionality at the beginning of the alkyl chain. Therefore, the polyol derived from an aromatic amine-based initiator is fully miscible with aromatic isocyanates and is unlike any polyol disclosed in either the '089 patent or the '659 patent.